In the extrusion of plastic material, and especially in making films, it is a consideration to maintain the heat-softened plastic as clean as possible. If there is any interruption to flow of the plastic material as is occasioned for changing the filter or screen plate, unwanted downtime results. In addition, many of these films are of such a nature that they require a very high degree of virgin plastic but if the filtering could be improved upon, a lower percentage of virgin material could be used, thus lessening the cost of the ultimate product.
The present invention provides an apparatus which does not require sensitive, accurate, heating of the screen inlet and outlet ports in order to actuate the filter change. The screen inlet and outlet ports remain at the same temperature as during normal operation of the machine. In the present apparatus there is no leakage of the material being filtered and the present apparatus reduces the necessary heat zone requirement to one zone only, that is, for the melt stream only.
All the suggestions heretofore have inherent weaknesses. In changing screens, the blown film tower is apt to collapse resulting in very costly down time and in the case of prior art continually changing endless filters, there have been operational difficulties which are apt to require shutting off the operation and rearrangement of the filter parts, there also having been faults in operation not heretofore corrected.
It is the object of the present invention to provide a continuous screen filter which is positively and mechanically moved in order to change a portion thereof across the breaker plate, i.e., the filtering portion thereof and to render the operation of filtering continuous avoiding any kind of down time and increasing the degree of filtration such that a lesser degree of virgin material can be used in the extrusion process.